Screw Conveyor - Reverse Engineer 1

[Doggert]

I have a screw conveyor that is oversized for what we need to do - but I'm not sure how oversized it is.

I need to determine the possible flowrate of this conveyor, but I'm unable to measure it directly without causing too many headaches.

I am able to take measurements of the actual screw itself, and I can measure the rotation of the screw. Is there a simple formula or procedure to follow to calculate what the flowrate would be from a screw conveyor of given dimensions + a given rotation? I'm assuming that flighting and screw diameter will factor in.

Thanks!

 

[eromlignod]

I assume you're talking about an ordinary augur-type dry product conveyor.

I don't know how accurate you need your figure, but if you assume that the flight is thin and most or all of the material is conveyed efficiently with each revolution, then it is a fairly simple problem.

Assuming you have a single-spiral flight, then you get the volume of one complete volume of flight with each revolution. If the flight is thin then this is basically the volume of one lead-length of tube, which would be the annular area of the tube (OD area - ID area of shaft) multiplied by one lead of the screw. The flow rate would be this volume multiplied by the rpm, yielding volume per minute.

Think of it like having a cylindrical segment of the tube exactly one pitch long full of water, then inserting the thin, helical flight. The water doesn't change much in total volume. Even though the confined water is now helical in nature, it is still basically the same cylindrical volume.

Don
Kansas City

 

[PeterCharles]

Maybe I could help if you posted the basic details of the screw :
Is it a "feeder" (is the inlet flooded with material)
Is it a "U" trough
Inclination of screw
Diameter of scroll
Diameter of tube
Pitch of scroll
Speed of rotation
Material being conveyed

 

[MintJulep]

I will share an unhelpful anicdote.

Years ago I was installing some equipment in a food processing facility in Mexico City. The line included overhead (hung from the roof structure) screw conveyors to distribute the material to some other equipment.

The crew trying to get the screw conveyors comissioned was having a difficult time.

1. Turn on screw.

2. Hear horrible screaches and grinding noise.

3. Lean ladder against the suspended trough, climb up with a big angle grinder. Straddle the trough.

4. Have someone else turn on the screw. Earball the spots where the screw is grating against the trough. Grind away the screw flights in the offending areas.

5. Get off the trough and climb down the ladder.

6. Restart from 1.

 

[Doggert]

Thank you for your replies - Don's idea is generally what I would have expected and sounds reasonable.

Thanks!

 

[macmet]

Why do you need to reduce the flow rate? Couldn't you just slow it down with a VFD or change the sprocket size?

 

[Doggert]

Because it was originally over-sized. The machine currently runs between 1% and 2% speed - which is ridiculous to control.

Also, it dumps piles of material into the process rather than a steady flow. This is because the flights and size of the screw are too large.

It's in a position where getting an accurate reading of flow would be nearly impossible - and in order to run it + the machine it feeds does not allow for an easy flow measurement either, since they combine with other flows before getting to a measurable point.

Thus, I'm left with determining the flow via calculation rather than measurement. Couple this with the fact that the original drawings/specifications of the screw are no longer available - and you have yourself a nice little engineering problem.

The response I recieved generaly agree with what I thought would be a good approach - I just wanted to check in with the experts for verification first.

Thanks!

 

[macmet]

Well, you aren't kidding about that being oversized.

 

[PeterCharles]

I'm still not clear about the type of screw we are talking about, is it a "bog standard" U-trough machine.

"..it dumps piles of material into the process rather than a steady flow.."

Pulsating outputs are a feature of various types of bulk material conveyors. In a conventional U-trough screw conveyor the percentage design filling is between 15% and 45% according to the bulk material being handled. As a result the output pulsates according to the speed of the screw flighting. The lower the loading the more obvious is the pulsation. There are various design features that can be added to a screw conveyor to minimise this.

If you want to know the actual output then an approximate method is to stop the conveyor under load, remove the cover and scoop out the material from, say, five flight pitches. From this you can determine the average mass per pitch which multiplied by the screw speed will give you the mass per hour.